The interval from delivery to the first fertile menstrual cycle in rhesus monkeys is not known. Breeding management requires such information to maximize the use of breeder males for space management and planning and efficiency in timed-mating protocols. Five nursing mothers and five non-nursing mothers will be bled daily for 90 days beginning one day after delivery. Serum levels of Follicle Stimulating Hormone, Leutinizing Hormone, Estradiol, Esterone, and Progesterone will be measured to identify the onset of ovulatory menstrual cycles.